Studies were undertaken to determine prevalence and natural behavior of type C RNA tumor viruses of cat, chicken, mouse and man. A method was developed for continuous culturing of mouse splenic lymphocytes. A type C virs which appears to be a new type of mouse type C virus was continuously released from cultures derived from BALB/c mouse but not from similar cultures of 4 other mouse strains. Evidence was obtained that feline sarcoma virus is similar to other mammalian sarcoma viruses in that FeSV is defective and depends on helper leukemia virus for synthesis of its viral glycoproteins. A method was developed for rapid serologic identification and typing of feline leukemia virus in field cats. The method involved identification of viral envelope antigens on cell surfaces of cat lymphocytes and feline cells infected in vitro by a membrane immunofluorescence technique. Horizontal transmission of feline leukemia virus was confirmed with use of feline leukemia virus with an envelope antigenic marker. A sensitive method was developed for rescue and demonstration of chicken endogenous virus in diverse genetic lines of chickens. Sero-epidemiologic studies of humans failed to suggest that a virus similar to woolly monkey sarcoma virus is etiologically involved in human malignancies.